I am trying to figure out one thing in IntelliJ IDEA 10:

Press Ctrl + Alt + Insert to disable the column selection mode.
Please note that this effect can also occur if you activate Column Selection Mode under: Edit -> Column Selection Mode which allows you to make square selections.
File | Settings (or Preferences on Mac) | Editor | Allow placement of caret after end of line:

You can set the Caret behaviour as below shown in the screenshot: